Guys who wear shirts emblazoned with "67" are sending secret, sexual messages to those in the know.  You might have already guessed the meaning -- the number 67 is two less than 69, implying that "It takes two to make a 69."  In other words, 67 plus two (as in two men) makes 69 (as in two men sucking each other off simultaneously).  So we're always "67ing" when we're not 69ing.  A shirt with 67 is saying, "I'm not 69ing right now because I need someone else's dick.  Two cocksuckers are required to go from 67 to 69.  Are you willing to make it all add up?"